This invention relates to a multipoint communication method and communication control device, and in particular relates to a multipoint communication method enabling communications between multiple points in a communication network, and a communication control device which realizes the above method.
In recent years there has been growth in transmission methods which perform transmission more efficiently than previously by combining such diverse media as voice, data, and images into packets, and transmitting in packet units over the same transmission channels, as seen in ATM, frame relay, and the Internet. Particularly in audio transmission, by making the transmission rate variable depending on the transmission channel characteristics (network traffic and error rate) and on the sound source, efficient transmission methods are being realized. For example, a mobile core network and the AMR (Adaptive Multi Rate) method for voice encoding in this network which are being studied for adoption of IMT-2000 are representative of systems with the above features. The AMR method is a method which determines the encoding/decoding rate according to the error rate and circuit conditions.
FIG. 13 is a diagram of the configuration of an IMT-2000 mobile core network, showing the case in which a mobile station MS communicates by voice with a telephone set, not shown, via a base station BTS, radio network controller RNC (Radio network controller) mobile service switching center MSC, and public switched telephone network PSTN. The mobile station MS incorporates a codec (coder/decoder, not shown); A/D-converted voice data is encoded at a rate indicated by the radio network controller RNC and transmitted, and encoded data which has been received is decoded into voice data, A/D-converted and output.
The radio access network RAN has decision rights for the AMR encoding rate; the radio network controller RNC issues a rate change protocol to the transcoder (TC) in the mobile switching station MSC on the core network (CN) side as necessary. As the AMR encoding rate, eight rates can be set in eight stages from 4.75 kbps to 12.20 kbps, as shown in FIG. 14; the encoding rate is specified using a rate specifier (RFCI). In IMT-2000, a plurality of encoding rates which can be used by the radio network controller RNC itself in order to provide free selection rights within the radio access network RAN, and the encoding rate currently in use, can be determined. That is, as shown in FIG. 15, the radio network controller RNC (1) sends the plurality of correspondence relations between rates and RFCIs (RFCS; RFCI set) to the transcoder TC at the time of negotiation with the transcoder TC in advance of the initiation of communication; (2) then, inputs the rate specifier RFCI into the transcoder TC, and effects encoding/decoding at the encoding rate corresponding to the RFCI; and, (3) dynamically changes the RFCI to change the encoding rate during communication in response to the error rate or other circuit conditions. FIG. 16 is an example of rate control information RFCS sent to the transcoder TC from the radio network controller RNC during negotiation, and shows that for RFCI=60 to 63 the encoding rate is 4.75, 5.15, 7.95, and 12.2 kbps.
The mobile service switch center MSC, which comprises a switch (not shown) and numerous transcoders TC, switches a packet input from a radio network controller RNC as appropriate via a prescribed transcoder TC, or without passing through a transcoder TC, for transmission to a public switched telephone network PSTN, and transmits packets from the public switched telephone network PSTN to the radio network controller unit RNC either via a transcoder TC, or directly. The transcoders TC incorporate a codec, and encode the PCM voice data input from the public switched telephone network PSTN based on the encoding rate specified by the rate specifier RFCI from the radio network controller RNC, then transmit the data to the radio network controller unit RNC, as well as decoding the encoded PCM voice data input from a mobile station MS and transmitting the data to the public switched telephone network PSTN.
As described above, the mobile station MS encodes voice data at the specified rate and transmits the data, which is input to the mobile service switching center MSC via the base station BTS and radio network controller RNC. A transcoder TC within the mobile service switching center MSC decodes the encoded voice data into PCM voice data, based on the rate corresponding to the rate specifier RFCI specified by the radio network controller RNC, and transmits the data to the public switched telephone network PSTN. PCM voice data transmitted from the other-party telephone set is input to the mobile service switching center MSC via the public switched telephone network PSTN. A transcoder TC within the mobile service switching center MSC encodes PCM voice data based on the rate specified by the rate specifier RFCI, and transmits the data to the mobile station MS via the radio network controller RNC and base station BTS. The mobile station MS decodes the input encoded voice data based on the rate specified by the radio network controller RNC, and outputs the decoded voice signals from a speaker.
Communication between the base station BTS and the public switched telephone network PSTN relies for example on ATM transmission; of this, communication between the base station BTS and the mobile service switching center MSC relies on AAL type 2 ATM transmission, and that beyond the mobile service switching center MSC relies on AAL type 1 ATM transmission. In mobile communication, in order to make efficient use of the communication band, data is compressed as described above to encode data in a low-bit rate data format for transmission. When such low-bit rate information is embedded in the payload of an ATM cell, time is required for the payload of one ATM cell to be filled with data, so that data delays occur and the quality of communication may suffer. AAL type 2 is suitable for low bit rate data transmission, and can be used to reduce delays and to efficiently utilize bandwidth. FIG. 17(A) is a diagram explaining the AAL type 2 format. An AAL type 2 format cell comprises a standard cell header HD and a standard cell payload PL; in the standard cell payload PL, there are mapped a one-byte start field STF and one or more short cells SCEL. The start field STF comprises a pointer (offset value) indicating the leading position of the first short cell SCEL. Each short cell SCEL comprises a fixed-length short cell header SHD and a variable-length short cell payload SPL; in the short cell header SHD, (1) PDU type, (2) frame number, (3) frame quality classification (FQC), and (4) an RFCI or similar, not shown, are embedded; in the short cell payloads PDU are embedded the above low-bit rate information (voice data), and appropriate rate control information RFCS.
AAL type 1 is a transmission method which realizes a CBR (constant bit rate) service; the clock timing on the receiving side is matched with the timing of the user clock (for example, the 64 kbps of voice data) on the transmitting side, and by this means the voice information on the transmitting side can be reliably reproduced on the receiving side. FIG. 17(B) is a diagram explaining the AAL type 1 format; the 48-byte standard cell payload PL comprises a one-byte SAR-PDU header and a 47 byte SAR-PDU payload. The 47-byte SAR-PDU payload is used to transfer PCM voice data; the one-byte SAR-PDU header is used for transmission and reproduction of user clock timing information.
The above rate control procedure is generally performed when making a connection between the public network PSTN and the mobile station MOBILE SERVICE SWITCHING CENTER MSC (hereafter called an L-M connection). In a connection between mobile stations (hereafter an M-M connection), a transcoder-free operation (TrFO) such as that shown in FIG. 18, which does not pass through a transcoder TC, is executed, to prevent quality degradation due to tandem connections while also effectively utilizing the resources of transcoders TC. That is, when a connection is made via transcoders TC1, TC2 (tandem connection), (1) encoded voice data from a mobile station MS1 is decoded into PCM voice data by a transcoder TC1 and transmitted; (2) the PCM voice data is encoded by the transcoder TC2; and (3) the encoded voice data is decoded into voice data by the mobile station MS2 and output from a speaker. However, in this method data passes through a transcoder (codec) twice, so that quality is degraded, and in addition numerous transcoders are required. Hence in an M-M connection, communication is performed using the TrFO method, without passing through a codec, and moreover transcoders are used only for calls from the public network PSTN.
In this TrFO method, because radio network controllers RNC1, RNC2 set their own rate control information RFCS, they may have different rate control information RFCS in a radio access network RAN. For example, as shown in FIG. 19, the radio network controller RNC1 sets the rate control information RFCS1 shown in (A), and the radio network controller RNC2 sets the rate control information RFCS2 shown in (B). In this case, each of the mobile stations cannot recognize the encoding rate indicated by the rate specifier RFCI sent from the other-party mobile station, so that encoded voice data cannot be decoded. Hence in the TrFO method, in the initialization performed prior to the start of communication, it is agreed that (1) rate control information RFCS1, RFCS2 is exchanged between the radio network controllers RCN1, RCN2 over the radio access network RAN, and (2) a rate specifier RFCI based on the rate control information RFCS1, RFCS2 issued by each to the other is transmitted as rate information. Under these conditions, each of the mobile stations can decode the encoded voice data sent from the other-party mobile station.
For example, if a mobile station MS1 has encoded voice data at a rate of 5.15 kbps, the radio network controller RNC1 appends a rate specifier RFCI of 61 to this encoded voice data and transmits the data. When the radio network controller RNC2 on the receiving side receives the rate specifier RFCI (=61) together with the encoded voice data, it refers to an RFCS1 table and to the received RFCI (=61), and determines that the received encoded voice data was encoded at a rate of 5.15 kbps.
The TrFO method in the above network results in no problems for voice communication between two mobile stations MS1 and MS2 (two-point communication). However, when a new mobile station MS3 is added to the two-point communication and an attempt is made at conversation among three mobile stations (three-point communication), the following problem arises. When realizing three-point communication, as shown in FIG. 20, the TrFO communication of the two-point communication that had been conducted up to that time must be interrupted (TrFO Break), and transcoders TC1, TC2 must be allocated to each of the mobile stations MS1, MS2, and a path inserted (the allocated transcoders TC1, TC2 are called “assigned TCs”). On the other hand, a transcoder TC3 is also allocated to the newly participating mobile station MS3, and three-point communication is conducted between the transcoders (FIG. 20). That is, the transcoder TC1 decodes the encoded voice data sent from the transcoders TC2 and TC3, synthesizes the decoded voice data, and encodes the synthesized voice data for transmission to the mobile station MS1. Similarly, the transcoders TC2 and TC3 decode the encoded voice data sent from each of the other two transcoders, synthesize the decoded voice data, encode the synthesized voice data, and transmit the encoded voice data to the mobile stations MS2 and MS3 to conduct three-point communication.
Here, the fact that the rate control information RFCS1 to RFCS3 is that of the radio access networks RAN1 to RAN3, and that during TrFO communication the core network CN does not possess this information, constitutes a problem. Consequently, the assigned TC1 and TC2 which are started upon a TrFO break cannot be provided with rate control information RFCS from the core network CN, so that if rate control information is not provided by some means, the assigned TC1 and TC2 will not have the rate control information RFCS1 and RFCS2, so that normal encoding/decoding cannot be performed. In light of this, when making a transition from two-point communication to three-point communication, it is necessary that negotiation of the rate control information RFCS1 (RFCS2) be performed between the radio network controller RNC1 (RNC2) and the assigned TC1 (TC2), by means of an initialization procedure similar to that used upon L-M connection, and moreover the RFCS1 and RFCS2 must be mutually negotiated between the assigned TC1 and TC2. However, because this negotiation takes time, a momentary interruption occurs in the voice communication (two-point communication) that had been in progress over the M-M connection. Further, in multipoint communication relying on transcoders, overhead relating to TC resources and to communication bandwidth between TCs occurs.